1. Field of the Invention
This invention relates to pellet loading devices, and, in particular, to a loader for picking up a rectangular-shaped, single layer, square lattice array of upright cylindrical fuel pellets and for placing the array of pellets into a container.
2. Description of the Prior Art.
Uranium oxide fuel pellets for use in nuclear reactors are formed by compressing uranium oxide powder into a dense right cylindrical-shaped pellet form having a packing fraction of about 95%, and then sintering the pellet. The sintering step is typically performed by loading several layers of the pellets in a rectangular-shaped sintering boat, wherein the upright pellets of each layer are adjacently packed in a square lattice array having a rectangular-shaped perimeter. The loaded sintering boat is then heated in a hydrogen atmosphere. An automated device used for loading the pellets into and out of the sintering boat includes a manifold and vacuum head connecting to a planar array of vacuum cups. The cups are lowered onto the top of a similarly arranged planar array of fuel pellets, wherein each pellet is standing on end on a table. Vacuum is simultaneously applied to each of the vacuum cups to enable the device to hold and lift all of the pellets in the array in unison. The array of pellets can then be placed on the bottom of the sintering boat as an array. The vacuum is then reduced, the array of vacuum cups is raised out of the sintering boat and the process is repeated to add more layers, with each additional array of pellets placed atop the previously placed layer in the sintering boat. An important design consideration for the loading device is to be able to maximize the number of pellets loaded into the sintering boat in each layer, while still being able to be cleanly inserted into and retracted from the sintering boat.
In recent years the nuclear industry has begun to use fuel rods that contain pellets having an annular-shaped cross section. Obviously, it would be difficult to pick up an array of such hollow core pellets with the prior art vacuum loading device. Presently, the only method of loading annular-shaped fuel pellets is by hand loading which is undesirable from a health physics standpoint. Therefore, there is a need for a new type of automated loading device that can pick up a square lattice array of cylindrical-shaped fuel pellets, each having an annular-shaped cross section, and place them in a sintering boat of the type currently used without requiring human handling and without damaging the pellets, wherein the number of pellets in the array is maximized.
In addition, present art pellet loaders are expensive and complicated. They require frequent cleaning because the vacuum lines connecting the vacuum head to the vacuum cups tend to clog with uranium oxide powder or other debris. The vacuum pumps must be qualified for use in the nuclear industry, adding further expense. Therefore, there is also a need for a less expensive and less complicated alternative to present art loading devices for use with solid cylindrical pellets.